1. Field of the Invention
This invention relates to a novel catalytic electrode in which the oxidation catalyst is protected from unfavorable redox potentials by an insulation/rectification matrix. Another aspect of this invention relates to a method of protecting electroactive materials from unfavorable redox potentials through use of such matrix.
2. Prior Art
Many substances, while stable at certain voltages, are extremely unstable at other voltages. Thus, these materials are extremely susceptible to swings in voltages. This problem is especially acute in bifunctional air electrodes which experience tremendous voltage swings in operating between the discharge and charge modes. For example, when operating in the charge mode, oxidation catalysts on the surface of bifunctional electrodes are relatively stable, however, when voltages are reversed and the electrode functions in a discharge mode, these catalysts are subject to reductive decomposition.
Air electrodes were first investigated at the turn of the century for use in fuel cells. More recently they have found application in metal/air batteries, such as zinc/air for railroad signalling and remote communications systems, aluminum/air and iron/air batteries for vehicular propulsion, and in fuel cells for utility power generation. Miniaturization has allowed entry by air electrodes into the hearing aid market. An air electrode operates on the reduction of oxygen to water as described by the reaction: EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O
Several advantages are attendant to the use of air electrodes in batteries. For example, batteries which contain air electrodes have very attractive energy densities relative to most other kinds of batteries because they utilize oxygen from the surrounding air as one electrode reactant. This results in a reduction in space and weight requirements normally associated with batteries in which cathode active materials must be stored within the battery. Consequently, battery systems which contain air electrodes can be compact and lightweight. In addition, use of oxygen from the air obviates present severe engineering and environmental problems encountered in advanced batteries based on molten sulfur, chlorine, bromine, and other corrosive cathode materials. With the incorporation of a bifunctional air electrode into a battery, there is no need for high temperatures, complex electrolyte systems and in the case of halogens, relatively complex plumbing and storage of the cathode material. The high energy density of air electrode batteries is of greatest advantage in applications which also require reversibility. Combination of these two characteristics in a single battery is of particular advantage in mobile systems that have weight and power restrictions. A bifunctional air electrode incorporated in a battery can operate both as a cathode, reducing oxygen to water for energy production; and as an anode, oxidizing water for energy storage. This reduces the overall weight while providing an environmentally safe power source in that only water and oxygen are produced.
Attempts to produce a commercially acceptable bifunctional air electrode have met with limited success due to a number of difficulties. One such difficulty is an inability to find a catalyst which functions well in both cathodic and anodic directions and which has sufficient survivability to the severe environmental conditions to which it is repeatedly subjected during cycling. One solution to this difficulty is to use a third electrode for recharging the battery. However, this solution results in other difficulties foremost of which is that it results in a complex and bulky system thus negating one of the desirable features of air electrodes.